The present invention relates to cement additive comprising two or more kinds of fluidizing agent for use in cement composition. More particularly, it is concerned with a cement additive which increases dispersibility of cement particles in a cement composition (cement paste, mortar or concrete), improves fluidity of the cement composition, and further prevents a reduction in the fluidity with a lapse of time, thereby increasing workability of the cement composition.
Because of high cohesive ability of cement particles in cement compositions such as cement pastes, mortars, and concretes, addition of the calculated amount of water for hardening of cement fails to provide a cement composition with high workability. In order to improve the workability of the cement composition, therefore, it is necessary to add a higher amount of water than the calculated amount. This will, however, lead to a reduction in the strength. Thus, in order to obtain the same strength while keeping good workability, it is necessary to increase the proportion of cement in the cement composition. When, however, the proportion of cement is increased, the amount of heat liberated during the process of hardening increases, yielding a disadvantage in that formation of cracks is liable to occur.
The cement composition, after preparation, is often transported to the working field by means of agitator trucks, for example. The time required for this transportation varies depending on the distance of transport, the degree of traffic delay, and so forth. Thus, the cement composition varies in fluidity at the working field from batch to batch, making it difficult to obtain a predetermined degree of workability. Moreover, in pumping under pressure the cement composition, if the pumping operation is interrupted for certain reasons and, after a certain period of time, is started again, the fluidity of the cement composition in the pipe will drop, producing various problems such as the necessity of increasing the pumping pressure after the restart of the pumping operation and partial plugging of the pipe.
The most convenient means to increase the fluidity of the cement composition is, as described above, to increase the proportion of water. Increasing the proportion of water, however, causes cracking and peeling-apart of concrete after the working. For this reason, it is desirable to use cement additives which permit reducing the amount of water added, increasing the dispersibility of cement particles, and further imparts suitable fluidity to cement compositions and enables keeping the suitable fluidity.
As such additives, a formalin condensate of sodium naphthalenesulfonate has heretoforce been known. This condensate, however, does not sufficiently satisfy the above-described requirements since although it can impart fluidity to the cement composition, it fails to keep the fluidity for a long period of time.